When an electric current is passed through a conducting liquid (electrolyte), what kind of change primarily takes place in the liquid?

Introduction / Context:
Passing electric current through liquids is a key idea in electrochemistry and is used in processes like electroplating, extraction of metals and electrolysis of water. Liquids that conduct electricity usually contain ions and are called electrolytes. This question tests whether learners understand that when a current passes through such a conducting liquid, it generally causes a chemical change rather than just a simple physical effect.

Given Data / Assumptions:
- The liquid mentioned is a conducting liquid, that is, an electrolyte containing ions.
- Electric current is passed between electrodes immersed in the liquid.
- We are concerned with the main effect on the liquid, not minor secondary effects.
- Typical school level examples include electrolysis of acidified water or copper sulphate solution.

Concept / Approach:
In electrolytes, current is carried by the movement of ions towards oppositely charged electrodes. At the electrodes, these ions either gain or lose electrons, leading to redox reactions. This process is called electrolysis and results in the formation of new substances such as gases (hydrogen or chlorine) or deposited metals (copper, silver). Since new substances are produced, the primary effect inside the liquid is a chemical change or chemical reaction, not just a physical change like heating or simple evaporation.

Step-by-Step Solution:
Step 1: Identify that the liquid is a conducting liquid, so it must contain ions that can move under the influence of an electric field. Step 2: Recall that when current passes through an electrolyte, cations move towards the cathode and anions move towards the anode. Step 3: Recognise that at the electrodes, these ions undergo reduction or oxidation, gaining or losing electrons, which produces new chemical species. Step 4: Understand that this process, electrolysis, is essentially a chemical reaction driven by electrical energy. Step 5: Conclude that the primary effect is a chemical reaction producing new substances, not merely a physical change or simple heating.

Verification / Alternative check:
Consider the electrolysis of acidified water. When current passes through the water containing a small amount of acid, bubbles of hydrogen gas appear at the cathode and bubbles of oxygen gas appear at the anode. These gases are new substances that were not present in gaseous form before the process. Similarly, in a copper sulphate solution electrolysis, copper metal is deposited on the cathode while the concentration of copper ions in the solution changes. Both examples clearly show that chemical changes occur as a result of the passage of current through the liquid.

Why Other Options Are Wrong:
Galvanisation refers specifically to coating iron with zinc and is not the general term for what happens in any conducting liquid. Evaporation may occur if the solution heats up, but it is not the primary or defining effect of current passing through an electrolyte. Saying only a physical change occurs ignores the formation of new substances at the electrodes. A small temperature rise may happen because of resistance heating, but it is a secondary effect. Therefore, options A, B, C and E do not capture the main chemical nature of the process.

Common Pitfalls:
A frequent misconception is to think of electric current only in terms of heating effects, like in an electric kettle, and to overlook chemical changes. Another pitfall is to confuse galvanisation and electroplating with all electrochemical processes, even though galvanisation is a specific corrosion protection technique. Students may also think that if they cannot see a colour change in the bulk solution, no chemical change has occurred. Remembering simple electrolysis experiments with visible gas evolution and metal deposition helps to keep the focus on chemical reactions in conducting liquids.

Final Answer:
When an electric current is passed through a conducting liquid, the primary effect is A chemical reaction producing new substances.